The invention relates to a photochromic composition, and more particularly to an organic photochromic composition comprising spiro [indoline-2,3'-[3H]-naphth [2,1-b] [1,4] oxazine](S.O.) dye and unconventional ultraviolet (UV) stabilizers.
Compounds which undergo reversible photo-induced color changes are termed photochromic compounds. When subjected to ultraviolet light or visible irradiation, these photochromic compounds change their transmission. They subsequently revert to their original color state when they are subjected to a different wavelength of radiation or when the initial light source is removed.
Although the organic photochromic materials have been known for over 50 years, they have not had widespread industrial or commercial use. This is primarily due to the irreversible decomposition phenomenon, generally known as light fatigue. Repeated exposure to light causes the photochromic materials to lose their photochromism.
It is thought that light or heat or both light and heat are responsible for the photodecomposition of organic photochromic compounds. Thus, many people have tried to increase the light fatigue resistance of the compounds by adding numerous conventional antioxidants or ultraviolet light absorbers. For example, U.S. Pat. No. 3,212,898 teaches the use of conventional UV absorbers such as benzophenone and benzotriazole to increase the photochromic life of photochromic benzospiropyran compounds. Similarly, U.S. Pat. No. 3,666,352 teaches the use of conventional UV light absorbers in photochromic mercury thiocarbazonate lenses, transparent to radiation of wavelengths greater than 4200 Angstrom units and opaque to radiation of wavelengths less than 4200 Angstrom units, in order to substantially increase the durability of the lense against photochemical degradation.
One class of organic photochromic compounds, spiro [indoline-2,3'-[3H]-naphth [2,1-b] [1,4] oxazine] dyes (S.O. dyes) are known to have good light fatigue resistance. In general, the light fatigue resistance of the spirooxazine compounds is about 100 times better than the closely related spiropyrans. This class of photochromic compounds has been disclosed in U.S. Pat. Nos. 3,562,172; 3,578,602; 4,215,010; 4,342,668; and 4,440,672.
The precise mechanism for photodecomposition of S.O. dyes is not yet fully understood. Although some circumstantial evidence indicates that oxygen is involved in the photodecomposition process, the traditional antioxidants (aryl amines and hindered phenols) do not improve the light fatigue resistance of S.O. dyes. The conventional UV stabilizers, substituted benzophenones and benzotriazoles, cause a small improvement in the light fatigue resistance of S.O. dyes, but they cannot be used effectively since they create a screening effect by absorbing UV radiation strongly in the region where the S.O. dyes absorb UV radiation. By competing with the S.O. dyes to absorb UV light, these conventional stabilizers substantially decrease the effective light intensity for S.O. dye activation. Furthermore, some of the conventional UV stabilizers are detrimental to S.O. dyes under certain conditions.
In addition, the presence of acids has a deleterious effect on S.O. compounds. Acids may be in the plastic host material from which the photochromic article is made, such as PVC or cellulosics. The acids are usually generated either thermally during the forming process, or photochemically during use of the photochromic article. The S.O. dye solution becomes a pinkish to reddish color in the presence of even a minute amount of acid, and the solution ceases to show a photochromic effect. The pinkish to reddish color is likely due to the formation of a complex between the S.O. compound and acid. A base must be used to neutralize the acid and to restore the original colorless or light blue color.
U.S. Pat. No. 4,440,672, incorporated herein by reference, discloses the use of organonickel complex stabilizers to improve the light fatigue resistance of the photochromic compounds. However, these organonickel complexes do not have the ability to neutralize acids which may be present.